This invention relates to a dynamo, and more particularly to a bicycle dynamo having a rotary-current generator which contains a stator and a rotor which can be rotated relative to the stator.
It is known that bicycle dynamos are used for feeding electric consuming devices arranged on the bicycle, specifically a headlight and a rear light, with electric energy generated by the wheel movement. The dynamos are arranged on the front wheel, for example, as side or spoke dynamos.
For the use on bicycles, for example, synchronous generators are known which have a single wire-wound coil on the stator and three permanent-magnet pole pairs on the rotor. Such single-phase bicycle dynamos have relatively high magnetic pole adherences. Another difficulty of-such simply constructed bicycle dynamos is the fact that, although, because of the only one used coil, they have a high internal impedance which permits a self-limiting of the supplied power, a failure of the rear light or of the headlight results in a noticeable voltage rise at the respective other, still intact light source which therefore becomes excessively stressed. In addition, at riding speeds below 15 km/h, these simple conventional bicycle dynamos do not yet furnish a power which is satisfactory for the operation of the bicycle lighting system, and in the case of high rotational speeds, the efficiency clearly decreases because of the formation of pronounced turbulent flows.
From German Patent Document DE 43 17 817 C1, a bicycle dynamo with a rotary-current generator is known in the case of which the stator consists of three identically constructed induction coils with pole finger cages made of soft iron, in which case the pole fingers of the individual pole finger cages are offset with respect to one another in the rotating direction by one third respectively of the distance of two adjacent pole fingers of the same pole finger cage. This has the purpose of achieving a reduction of the magnetic pole adherence, that is, of the pole sensing ability, in that, in each case, only one stator cage can be in a full magnetic force connection while the respective other two stator cages partially take up a neutral position as the result of subtracting magnetic tensile forces on the pole fingers.
There is a need for a vehicle dynamo with a rotary-current generator which has a low pole sensing ability; even at low riding speeds, provides a relatively high electric output power; and can be produced by means of expenditures which are acceptably low for the field of bicycle electric systems.
These and other needs have been met by the present invention by providing a bicycle dynamo comprising a rotary-current generator including a stator and a rotor which can be rotated relative to the stator, one of the stator and the rotor having a first number of radially extending pole fingers which are individually wound with one surrounding magnetic coil winding respectively, the other of the stator and the rotor having a second number of poles, a ratio of the first number to the second number being a non-integer value, and a ratio of the second number to the first number being a non-integer value.
In the case of this dynamo, the stator and/or the rotor has radially extending pole fingers which are individually wound by means of one surrounding magnet coil winding respectively. The ratio of the number of poles of the rotor to the number of poles of the stator is a noninteger value, and the ratio of the number of poles of the stator to the number of poles of the rotor is a non-integer value. This non-integer pole ratio keeps the pole sensing ability of the dynamo low. The radially extending pole fingers and their individual winding with one magnet coil respectively permit the implementation of a rotary-current arrangement which, even at low riding speeds, that is, at low rotational rotor speeds, is able to provide a comparatively high electric power. In this case, the individual winding of the pole fingers on a correspondingly designed special machine can be carried out very economically and permits a high filling ratio of the grooves between the pole fingers. The bicycle dynamo which is designed in this manner, even starting at low riding speeds in the range of from 5 km/h to 10 km/h, has a remarkably high efficiency so that the increased electric power does not require any noticeably higher expenditures of force.
According to a further aspect of the present invention, the bicycle dynamo is further optimized by arranging the stator surrounding the rotor in a ring-shaped manner, the stator having twelve pole fingers which extend radially to the inside and are wound by one surrounding magnetic coil winding respectively, the respective magnetic coils of each third pole finger being connected in series from a star point, and the rotor containing a permanent magnet made of a plastic-bound neodymium iron boron material with eight circumference-side magnetic poles. This embodiment has particularly low losses in the no-load as well as in the load operation which, in addition to being the result of the special individual pole finger winding, mainly results from the specifically selected pole number ratio, the selection of a laminated stator preferably made of high-quality dynamo sheet metal as well as the use of a plastic-bound neodymium iron boron material for the rotor.
According to a further aspect of the present invention, a voltage-limiting switching circuit part is provided in the load current circuit which lets the rectified output voltage pass through unaffected below a selected limit voltage whereas, in the case of an input voltage which is above the limit voltage, it controls the voltage at the output of the switching circuit part to the limit voltage. In this manner, the electric supply voltage, for example, for the bicycle lighting, remains at a constant value at a rising riding speed, and, when the rear light or the headlight fails, there is no danger of an excessive loading of the other, still intact light source.
According to a further aspect of the present invention, the voltage-limiting switching circuit part contains a current limiting circuit which controls a switch of the switching circuit part in a current-breaking manner as soon as a given limit voltage is exceeded by way of an intermediate resistance, which limit voltage is selected such that a torque produced by the generator is limited to a value which prevents a slipping of a torque transmitting part provided on an input side of the generator. This embodiment has the advantage that, as the result of the current-limiting switching, not only the dynamo is protected from an overload and a short-circuit, but mainly also the generator torque is limited in order to prevent a slipping of a mechanical torque transmitting part, for example, a toothed belt or a roller, provided on the generator input side.
According to a further aspect of the present invention, a field effect transistor with an excess temperature switch is used as the power switch of the switching circuit part, whereby, at a high bicycle speed, in the case of a short circuit, an overheating of this switch is avoided.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.